The present invention relates generally to wireless telecommunication networks. More specifically, the invention relates to techniques for obtaining cell related information in an overlaid network providing circuit switched and packet switched services.
The evolution of wireless telecommunication systems has led to continuing improvements in quality, capacity, and added functionality as compared to prior generations. One area of added functionality has been the introduction of packet switched data services such as General Packet Radio Service (GPRS) specified for use with the Global System for Mobile Communications (GSM) wireless standard. The implementation of packet switched services in wireless telecommunications allows for the introduction of a number of new data based services in current generation systems, e.g., enhanced support for Internet communications. Packet switched techniques provide a significant improvement in efficiency for utilizing radio resources relative to conventional circuit switched techniques.
Packet switched connections are an especially suitable means for transmitting data and voice over the air interface since radio resources are only reserved when the data or voice, which are divided into packets, need to be sent. Resources are relinquished and made available to other users during breaks between the packets thereby freeing up radio spectrum during periods of non-use. Further gains in efficiency are achieved by sharing resources among users by having multiple users share the same radio channel, i.e. frequency and time slot. In contrast, conventional circuit switched techniques allocate radio resources for the entire duration of the transmission. A comprehensive description of GPRS and packet data communications is presented in PCT publication WO/9516330 entitled: xe2x80x9cApparatuses and Mobile Stations for Providing Packet Data Communication in Digital TDMA Cellular Systemsxe2x80x9d, published on Jun. 15, 1995 with a priority date of Dec. 10, 1993 and assigned to the present assignee, the disclosure of which is incorporated here by reference. An example in the trend toward wireless evolution has been the development of the Enhanced Data Rates for GSM Evolution (EDGE) which is currently under standardization within the European Telecommunication Standards Institute (ETSI). The EDGE specification has also been selected by the Universal Wireless Communications Consortium (UWCC), as the outdoor high speed data component of UWC-136, which gives a common evolution path for IS-136 Digital Advanced Mobile Phone System (D-AMPS) and GSM to support high-bit-rate wireless data services such as simultaneous voice, internet services, and video at transmission up to speeds of 384 kbps (48 kbps per timeslot).
The EDGE specifications have been developed so as to offer packet data communications at high speeds within current frequency bands and is based on the current GSM Time Division Multiple Access (TDMA) frame structure, logical channel structure and 200 kHz carrier bandwidth. The current GSM and D-AMPS (which air interface standard provides for a 30 kHz carrier bandwidth) installed base stations can be upgraded in a step-wise manner for a gradual evolution to the higher bit rates. This enables systems operating in accordance with both network standards to achieve improved bit-rate performance within current frequency allocations and in existing cell plans.
In an integrated D-AMPS/EDGE network having circuit switched and packet switched functionality, EDGE channels applied in a GPRS network may be overlaid on top of an existing D-AMPS network. For example, the EDGE/GPRS portion of the network may support packet-switched (e.g., data) communications, while the D-AMPS portion of the network may support circuit-switched (e.g., voice or fax) communications. These system portions, however, have different operational characteristics. For example, EDGE, including traffic and control channels, uses a radio interface based on a 200 kHz carrier bandwidth wherein D-AMPS uses 30 kHz carrier bandwidth. Problems can arise from the operation of a dual-mode mobile station (MS) in an overlaid 30 kHz and 200 kHz network.
By way of example, in such a system an idle, dual mode MS typically camps on the packet-switched portion of the network by monitoring the 200 kHz common control channels (PCCCH or CCCH). Thus, an incoming page message for a circuit switched voice call to the MS is normally forwarded by the GPRS network over the PCCCH or CCCH prior to the MS setting up the call on the D-AMPS portion of the system. In the same way, other system related messages, e.g., registration, can be handled through the GPRS network.
Under these circumstances, a problem may arise from the fact that, in overlaid systems, the 200 kHz GPRS defined cells may not necessarily correspond geographically to the 30 kHz cells. Thus there is no way for the MS to know precisely which 30 kHz cell it is in when the page is received, since it has not been monitoring the 30 kHz digital control channels (DCCHs) associated with the circuit-switched portion of the network. Accordingly, the MS does not have the opportunity to obtain the cell specific information necessary for correct communication with the 30 kHz network. This scenario is particularly likely after the MS has moved around between EDGE/GPRS cells thereby increasing the likelihood that the MS may lose track of its current 30 kHz cell location, for example.
The lack of cell specific information associated with such overlaid systems affects a number of processes such as cell selection, cell reselection, and registration (location updating). Cell selection is the process by which the MS selects the most appropriate cell in which it can reliably camp on. Cell reselection is the process by which the MS checks that it is camped on the most appropriate cell and reselects a more appropriate cell when necessary. The most appropriate cell is typically determined by performing signal strength measurements on the serving and neighboring cells such that camping occurs on the cell providing the best radio conditions. However, those skilled in the art will recognize that factors other than signal strength, e.g., different types of service capability, can be considered as part of the process of identifying the most appropriate cell. In this regard, the interested reader is directed to U.S. Pat. No. 5,778,316, entitled xe2x80x9cA Method for Communicating in a Wireless Communication System, to Raith et al., the disclosure of which is expressly incorporated here by reference.
Registration is the process by which the MS informs the system of its current geographical location. The cell specific information for the 30 kHz network that is broadcast over the DCCH is required for the registration process, e.g. location updating when roaming, and this information is typically not available to the 200 kHz network. Without accurate cell information regarding the 30 kHz network, MSs camped on the 200 kHz packet switched network will not be able to reliably execute circuit switched calls over the 30 kHz network.
One solution which has been described for dealing with this problem can be found in U.S. Pat. No. 5,353,332 entitled xe2x80x9cMethod and Apparatus for Communication Control in a Radiotelephone Systemxe2x80x9d to Raith and Muller, the disclosure of which is also incorporated here by reference. In this patent, a control channel can contain both absolute information regarding characteristics of that cell and relative information regarding the characteristics of other, e.g., neighboring cells. This enables, for example, an MS reading that control channel to determine which cell is appropriate for selection or reselection.
However, the techniques described in this latter Raith patent may not be readily employable across different system standards and types of systems, e.g., circuitswitched and packet-switched systems. Thus, it is an objective of the present invention to provide a technique for obtaining the necessary cell information for the reliable operation of an overlaid circuit switched and packet switched network.
Briefly described, and in accordance with a number of exemplary embodiments thereof, the invention discloses a method of obtaining cell specific information for a dual-mode mobile station (MS) operating in an overlaid 30 kHz and 200 kHz circuit switched and packet switched network. In accordance with a first embodiment of the invention, cell reselection measurements are performed on both 30 kHz and 200 kHz control channels while the MS is camping or in transfer packet mode on the 200 kHz network. For the 200 kHz cell reselection, standard General Packet Radio Service (GPRS) procedures are used to perform measurements on the 200 kHz BCCH carrier. For cell reselection on the 30 kHz network, the MS then tunes to a selected digital control channel DCCH to obtain cell specific information. After completing scans on both networks, the MS is then armed with the information necessary to respond to an incoming page from the 30 kHz network.
In accordance with a second exemplary embodiment, the MS performs a scan and e.g. performs signal strength measurements to select a DCCH on the 30 kHz network exclusively. If a new DCCH is selected, the MS then tunes to the selected DCCH, reads the cell specific information associated with the 30 kHz network and then scans to select a broadcast control channel (BCCH) on the 200 kHz network. The MS tunes to and decodes the selected BCCH to obtain cell specific information. The MS then tunes to and camps on a PCCCH or a CCCH indicated in the cell specific information. In this way, the MS has acquired the necessary information to respond to a page from the 30 kHz network while camping on and being able to respond to a page from the 200 kHz network.
In a third embodiment, the MS scans exclusively the 200 kHz control channels i.e. the BCCH as defined in GSM. When the MS, camped on 200 kHz packet mode, receives a page from the 30 kHz network via the 200 kHz network, it then scans for selecting a DCCH on the 30 kHz network. The MS then reads the associated cell information before responding to the page. Hence, MS is able to obtain all necessary information in order to respond to a incoming page from the 30 kHz as well as from the 200 kHz network.
A fourth embodiment describes an overlaid 30 kHz and 200 kHz network having coordinated cell planning i.e. all cells in the 30 kHz network correspond to those in the 200 kHz network. In this case, cell selection and reselection can be accomplished by scanning the control channels of either the 30 kHz network or 200 kHz network. If, for example, the MS chooses to scan the 30 kHz DCCH, the MS in addition to reading the cell information, also reads a pointer to the 200 kHz BCCH contained on the DCCH. In the reverse case where the MS scans the 200 kHz BCCH, the MS reads the cell information and a pointer to the 30 kHz DCCH.